Known systems for treatment of liquids, such as aqueous waste effluent from laboratory and industrial facilities, e.g., chemistry laboratories, hospitals, biotechnology research laboratories, industrial processes and the like, must handle incoming wastewater streams which vary over time both in flow rate and in composition. The composition may vary either as to the chemical constituents in the wastewater, the concentration of constituents or both. The liquid which must be treated from facilities dealing with chemical and/or biological materials frequently is intermittently quite acidic or alkaline. Such treatment systems typically employ a batch reactor or a continuous stirred tank reactor (CSTR). In a batch reactor, a liquid is accumulated in one of two or more treatment tanks or in a temporary holding tank. The liquid is treated in the batch reactor until expiration of a preset time limit or attainment of preset conditions triggers discharge. Such discharge may be either to another treatment stage or final discharge to storage, a sewer or other final destination. Batch reactor systems are disadvantaged in that they typically require greater holding volume than do CSTR systems. At least two tanks are required, one for treating a first batch of wastewater while a holding tank or a second treatment tank is receiving a second treatment batch. A batch system generally is more expensive for a given treatment capacity in view of the need for holding tanks and/or multiple reactor tanks. In certain installations, particularly where existing facilities are being retro-fitted with a liquid treatment system, available space may not accommodate multiple reactor tanks.
In a CSTR system, a reactor tank is continuously open to accept an incoming flow of the liquid. A CSTR reactor also is adapted typically for continuous discharge of treated liquid. In fact, for certain applications, such as treatment of aqueous waste effluent from a hospital, laboratory, biotechnology research or production facility, etc., such treatment systems frequently are required to have a discharge outlet which is open full time during operation of the system.
CSTR systems are advantageous in that wastewater or other liquid is continuously treated and discharged. CSTR systems are for this reason particularly suitable for treatment of variable flow liquid in situations in which applicable regulations prohibit closing the flow of liquid into or out of the treatment system. CSTR systems are disadvantaged, however, in that a sudden spike in the flow rate of the in-flowing liquid will reduce the average or effective residence time of the liquid in the treatment tank, possibly causing the discharge flow from the reactor tank to exceed process or regulatory limits for a given characteristic or property, such as pH, oxidation reduction potential ("ORP"), dissolved oxygen ("DO"), etc. Similarly, a sudden increase in concentration may result in the effective residence time being insufficient to adequately treat the liquid, given the treatment system's capacity.
Those who are skilled in this area of technology will recognize that in a typical CSTR system, the effective residence time or the average or mean residence time of the liquid in the reactor tank is a function of the holding volume of the tank and of the rate of influx of liquid. The discharge flow from the CSTR reactor tank is a stirred mixture of old and new liquid (plus any added treatment materials) which, at any point in time, has an average bulk property which should meet process or regulatory requirements. Thus, the holding volume of the tank contains a body of liquid undergoing treatment. A given CSTR treatment system, faced with a sudden increase in influx rate (e.g., above its rated steady state capacity and/or concentration), may simply be unable to feed reagent (for example, an acid or alkali reagent for bringing the pH of influx wastewater to within a prescribed range set by applicable regulations or control limits) at a sufficiently high feed rate, given the system's maximum pumping rate for the reagent and the effective residence time of the system at that influx rate, for adequate treatment. Thus, for a time following such sudden influx, the discharge stream from the reactor tank may fail to comply with applicable regulatory or process control limits. In certain instances, such lack of compliance may comprise a civil or criminal violation and may result in imposition of a fine or other sanction.
It is known to use multiple stage CSTR reactor tanks to deal with the problem of variable influx streams, wherein the volume and/or concentration of the stream may periodically exceed, for short periods, the treatment capacity of the first CSTR reactor tank. As discussed above, however, the use of multiple reactor tanks can significantly increase both the cost and the space requirements of the system, often negating the benefits of such CSTR system over a comparable batch treatment system. A desirable treatment system for liquid, such as aqueous waste effluent mentioned above, having a full-time open discharge during operation, would include the space and cost benefits of a CSTR system with the capacity of a batch treatment system for handling variable concentration and flow rates. It is an object of the present invention to provide a CSTR system which meets this need. Additional objects and advantages of various embodiments of the invention will be apparent from the following disclosure.